Energy demand and energy efficiency of telecommunications networks have recently become a greater center of attention. In view of the critical requirement of sustainable use of resources it will also be expected in future that an energy-efficient network operation will be indispensable for telecommunications network operators and telecommunications service providers.
In particular access networks have a large share in the energy demand of today's telecommunications networks since the network elements are dispersed over a wide geographical space and the network has a great number of active components—at least at the networks' boundaries. Passive Optical Networks, PONs, are considered by many network operators to be the next step in the further development of access networks and are currently further expanded in order to ensure future-proof optical access networks for high-bit usage.
It is assumed that at first the generation of the Gigabit-capable PON [2] and subsequently the also already standardized variant XG-PON1 [4] will be used. These PON variants are based on an Optical Distribution Network, ODN, which uses as central element an optical power coupler which is wavelength-independent and distributes the signals from the central node uniformly—i.e., not frequency or wavelength selective—to all end nodes. For the future of line-based optical access networks going beyond the XG-PON1, PON variants are discussed which use the Wavelength-Division Multiplexing, WDM, in order to be able to provide per end node higher transmission bit rates per connection by providing exclusive wavelength channels. Among the various variants discussed are such of particular interest which can still use the mentioned optical distribution network—further developed for G-PON and XG-PON1—and consequently continue to use a passive optical power coupler as well. Thus, the partly great financial resources required for a nationwide expansion of fiber optic access networks can be guarded over a longer period of time. Such a method is the UDWDM-PON based on coherent superheterodyne receivers on which the present description of the invention is based [1].
Published references referred to herein:    [1] H. Rohde et al, “Next generation Optical Access: 1 Gb/s for Everyone”, in Proceedings European Conference on Optical Communication (ECOC) 2009, Paper 10.5.5.    [2] Keiser, Gerd: FTTX—Concepts and Applications. Hoboken, N.J.: Wiley, 2006 (pages 13-14)    [3] ITU-T Series G. Supplement 45: Means and impact of GPON power saving. White Paper, Genf, 2009    [4] ITU-T Series G, G.987: 10-Gigabit-capable passive optical networks, Geneva 2010    [5] E. Gottwald, H. Rohde, S. Smolorz, “Machbarkeit kohärenter Einfaser-UDWDM PONs mit 1000 Teilnehmern und 100 km Reichweite”, 11. ITG Fachtagung Photonische Netze, Paper 12, 05/2010.    [6] Klaus Grobe, Jörg-Peter Elbers: PON in Adolescence: From TDMA to WDM-PON. IEEE Communications Magazine, pages 26-34, January 2008